Any active domain name that's added in a hosting account includes a number of records which allow it to work properly. In the standard case, the name servers (NS records) show the location where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are particular records set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a particular port number such as Voice-over-IP), etc. By having access to these records and by being in a position to modify them, you could have your website with company A and your e-mail messages with company B, for example. This gives you more independence as you can choose the best provider for every single one of these services and you do not have to stick to a single company. Whenever you select a hosting company, you have to make certain that you have access to all DNS records freely because some companies have an additional fee for this function.